When a large display or part of a display, colour filter or other similar application, is produced, an exposure system transfer an image from a glass plate, preferably made from high quality quarts, onto a rather large substrate, which may have a dimension up to 1100 mm times 1300 mm or even more. The exposure system includes an aligner, or stepper, that emits light through the glass plate and onto the substrate, see FIG. 1. The glass plate is held in place by two rulers, or alternatively by a frame, and therefore the shape of the glass plate is deformed and the aligner, or stepper, compensates for this calculated deformation. The front side of the glass plate that carries the pattern of the image is arranged on the rulers, and a perfect reproduced image by the system on a substrate is dependent on that the front side of the glass plate is absolutely flat.
It is very important that the registration of masks, i.e. the absolute placement in a Cartesian coordinate system, is good enough to permit masks from different systems to fit together, e.g. the colour filter and the TFT-array. Furthermore, large TFT substrates may use two or more masks stitched together to cover a large exposure area.
In pattern generating systems for small plates, a three-foot device is used to support the plate during pattern generation and measurement, but the weight of a glass plate, with a thickness of 10 mm and a size of 1000×1000 mm, is approximately 40 kg, which will not be suitable to place on three pins. An alternative solution is to use an air cushion for plate support, but this introduces other problems like determining the exact position of the plate during exposure of the pattern. Another alternative is to handle the consequences that will arise when placing the plate directly on the stage (i.e. the support) of a pattern generating apparatus, although the plate will be deformed.